– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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International metrology in the field of Time and frequency

The foundations of time and frequency metrology are:

  • the realization of the SI unit of time, based on an atomic transition with an accuracy at the level of a few parts in 1016, and

  • the construction and dissemination of atomic time scales based on the SI second.

Most people take it for granted that you can find out the time by looking at a clock. Many will even have checked the accuracy of their clock at home against a reference such as their mobile phone network. But for applications where space-time references are particularly important – such as satellite communications for example – it is necessary to know the time to a much higher level of accuracy... How does anyone know what the time really is?

In fact "the time" is calculated for the world by the BIPM, based on data collected from the world's most accurate clocks, which are developed by national laboratories to realize the SI second at the highest level. Collectively these clocks allow the construction of the international reference time scale (UTC), and individually they also provide essential tools for studies of fundamental physics.

The world today relies on time synchronization at all levels of precision; from the minutes or seconds needed by the general public to the nanoseconds required by an increasing number of demanding applications. Global navigation satellite systems (GNSS) rely on precise time to enable precise ranging measurements for positioning. The GNSS services are critical to the infrastructure of the modern world, for communications systems of all kinds, from satellites to cell phones, power-grid synchronization, financial transactions, scientific applications, etc. Accurate time is today disseminated by terrestrial and space navigation systems, through computer networks, telecommunication networks and the variety of other systems they serve. Digital time services operate all over the world, and are mostly used for commercial and financial operations. Trusted time stamping systems have been standardized in many countries for providing traceable time stamps in digital documents.

Time and frequency
SI unit of time (second)
Committee structure
What time is it?
Reference time scales
Strategic plan
International equivalence

At the BIPM
BIPM technical work
BIPM calibrations for time transfer
BIPM Annual Report on Time Activities
Recent BIPM publications in the scientific literature
FTP server of the BIPM Time Department
BIPM Time Department Database
BIPM liaison and coordination work
IERS Conventions Center
BIPM contact person